This invention relates to a system for controlling the flow of current to windings used in rotating machinery, and more particularly to controlling the flow of current to superconducting windings. The application also relates to telemetry electronics for exchanging information with, and about, the system.
Superconducting windings are being used in electrical machinery and rotating machines because of their low loss characteristics. While the superconducting windings are maintained at cryogenic temperatures, the power supplies used to drive the superconducting windings are typically maintained at ambient temperatures (300xc2x0 K.).
In the design of electrical machinery, incorporating high temperature superconducting (HTS) windings (i.e., motors, generators, magnets), the heat leak associated with the leads carrying current from the power supply at ambient temperatures to the cryogenically cooled windings is an overriding design factor which dictates the cost and thermal capacity of closed-cycle cryogenic cooling apparatus. These losses increase as the temperature difference between ambient and coil temperature increases. A number of approaches have been suggested to minimize the impact of heat leaks in such systems especially those in which the leads carry currents approaching 1 KA. Unfortunately, where vapor cooling of leads is not an option, these approaches introduce high voltages into the system or do not eliminate the need for a high current lead pair entering the cryogenic environment with attendant heat leaks. In cases where the superconducting coil is rotating with respect to a warm stator coil, the problem of heat leaks into the cryogenic environment becomes more critical due to the design constraints imposed by the thermal path impedance of a stationary cryocooler coupled indirectly to a rotating heat load or constraints on the size, weight, and thermal capacity of a rotating cryocooler.
There exist a number of large scale commercial and defense applications of HTS coils (e.g., magnet systems, generators and synchronous motor field windings) which require relatively constant magnetic fields, and in which ample time is available to ramp the coil current up to its initial desired value prior to regulated operation. In electrical machine systems incorporating HTS windings, the current in the HTS coil is subject to flux creep due to the finite losses in the HTS conductor. The dissipation due to this finite, albeit small, resistive loss requires that the current be restored periodically, i.e., xe2x80x9cpumpedxe2x80x9d via regulating circuitry back to its desired level. The energy input requirement is only that required to make up for the flux creep. Electronic circuits and mechanisms, which perform these functions, are referred to as xe2x80x9cflux pumpsxe2x80x9d.
The invention features an exciter assembly and approach for supplying power to a superconducting load, such as a superconducting field coil, disposed within a cryogenic region of a rotating machine. The exciter assembly provides an efficient and reliable approach for transferring the electrical power energy across a rotating interface and for controlling the ramp up and regulation of field excitation current in the field coil. In particular, the invention provides telemetry circuitry that allows local and remote devices to communicate with the exciter assembly and vice versa.
In general, in one aspect, the invention features an exciter assembly that supplies current to a superconducting load. The exciter assembly includes a transformer for generating the current and an optical emitter and an optical receiver. The transformer includes a stationary winding portion having a stationary winding and a rotatable winding portion having a rotatable winding that outputs the current for the superconducting load. The optical emitter and the optical receiver define an optical path over which information is exchanged between the stationary winding portion and the rotatable winding portion. Using optical communications is advantageous because it reduces interference from external sources relative to other communication methods.
This aspect may include one or more of the following features. The stationary winding portion and the rotatable winding portion may be concentric rings or facing surfaces. The assembly may include a controller and circuitry for controlling the current supplied from the transformer to the superconducting load. This circuitry may be coupled to the rotatable winding portion. The circuitry exchanges information with the controller over the optical path. The controller includes an interface to an external device, over which the controller provides information to, and receives information from, the external device. The external device may be a device on a network. At least some of the information transmitted between the controller and the external device and between the controller and the circuitry may be the same information.
The optical emitter may be on the stationary winding portion and the optical receiver may be on the rotatable winding portion. The optical emitter and the optical receiver provide first information from the controller to the circuitry. The exciter assembly also includes a second optical emitter on the rotatable winding portion and a second optical receiver on the stationary winding portion. The second optical emitter and the second optical receiver provide second information from the circuitry to the controller. The first information includes at least one of a command to maintain a level of the current in the superconducting load and a command to release energy from the superconducting load. The second information includes diagnostic information that relates to one or more of the circuitry and the superconducting load.
The optical emitter and the optical receiver emit and receive, respectively, one of infrared light and visible light. The assembly also may also include a power source which supplies initial current to the stationary winding. The transformer generates the current in the rotatable winding from the initial current.
In general, in another aspect, the invention features an exciter assembly for supplying current to a superconducting load. The assembly includes a transformer for generating the current. The transformer includes a stationary winding portion having a stationary winding and a rotatable winding portion having a rotatable winding. A first optical emitter is mounted on the stationary winding portion and a first optical receiver is mounted on the rotatable winding portion. The first optical receiver receives information via light having a first wavelength from the first optical emitter. A second optical emitter is mounted on the rotatable winding portion and a second optical receiver is mounted on the stationary winding portion. The second optical receiver receives information via light having a second wavelength from the second optical emitter.
This aspect may include one or more of the following features. The first and second wavelengths of light may be different. For example, the first wavelength of light may be an infrared wavelength and the second wavelength of light may be a visible wavelength. The first wavelength of light may be a visible wavelength and the second wavelength of light may be an infrared wavelength. The first and second wavelengths of light may be part of the same region of the electromagnetic spectrum. For example, the first and second wavelengths of light may both be infrared wavelengths. The first and second wavelengths of light may both be visible wavelengths. Carrier frequencies of the light having the first and second wavelengths may be between 30 kHz and 1 MHz.
The stationary winding portion and the rotatable winding portion may be portions of concentric rings that cover greater than 180xc2x0 and that overlap at least in part. The stationary winding portion and the rotatable winding portion may be facing surfaces. The stationary winding portion may include an air core to support the stationary winding and the rotatable winding portion may include an air core to support the rotatable winding.